1. Field of the Invention
The present invention relates to a method of regenerating a spent pickling solution composed of nitric acid and hydrofluoric acid and containing metal fluorides and metal nitrates. In accordance with the method, the pickling solution is heated in a reactor to about 300-1,000xc2x0 C., the metal fluorides and the metal nitrates are pyrohydrolized, and the metal oxides formed as a result are removed as a granulate, and the gas produced in the fluodized bed reactor is dedusted, cooled and fed to an acid recovery unit.
2. Description of the Related Art
DE-OS 43 15 551 discloses a method of regenerating a spent pickling solution composed of nitric acid and hydrofluoric acid and containing metal fluorides and metal nitrates, in which, after carrying out upgrading, for example, in a cycle, the spent pickling solution is introduced into a fluidized bed reactor which has been heated to a temperature of about 500-1,000xc2x0 C. and in which a fluidized bed is formed, for example, of iron oxide granulate. In the reactor, the metal fluorides contained in the spent pickling solution are pyrohydrolized. Subsequently, the acid contained in the flue gas leaving the fluidized bed reactor is recovered by absorption of hydrogen fluoride and nitrogen oxides. During the pyrohydrolysis, the nitrate is reduced to nitrogen oxide, which means that the recovery of the nitric acid portion is relatively cumbersome. In addition, relatively expensive oxidizing agents are required. Moreover, the degree of recovery still needs improvement.
Therefore, it is the primary object of the present invention to further develop the above-described method in such a way that the efficiency of the nitrate content and fluoride content in the form of free acid in the spent pickling solution or in the mixed acid is improved, and the recovery of the nitric acid can be carried out without the use of oxidizing agents and, thus, with lower operation costs. In addition, the method should continue to have the advantage that the dissolved metal irons can be transferred into a dust-free and recoverable form which can be treated well and is environmentally without problems.
In accordance with the present invention, in a method of the above-described type, the spent pickling solution is initially separated in a separating process into a partial flow with free acid and into a partial flow enriched with metal salts, and the partial flow enriched with free acid is fed to the acid recovery unit and the partial flow enriched with metal salts is fed to the fluidized bed reactor.
The method according to the present invention makes it possible to recover approximately 99% of the fluoride portion and approximately 90% of the nitrate portion in the form of a directly reusable mixed acid. Oxidizing agents are no longer required. Waste water flows are no longer produced, so that no neutralizing chemicals are used and no metal hydroxide sludges must be disposed of. The quantity of fresh acid required for topping up the pickling solution can be substantially reduced. Approximately 95% of the metals contained in the spent pickling solution are transferred into a dust-free and recoverable state which can be treated easily and is environmentally without problems.
The various features of novelty which characterize the invention are pointed out with particularity in the claims annexed to and forming a part of the disclosure. For a better understanding of the invention, its operating advantages, specific objects attained by its use, reference should be had to the drawing and descriptive matter in which there are illustrated and described preferred embodiments of the invention.